The present invention relates to an improvement of the fixation device for fixing a toner image onto a recording material such as paper in image recording apparatus such as electrophotographic copying apparatus, facsimile, and the like.
In electrophotographic copying apparatus, contact-type heat roller fixation devices are mostly used for fixing an unfixed image onto a recording material such as recording paper. In a process of this type, an unfixed toner image-bearing recording material is transported into between a pair of revolving bodies, a heat roller and a pressure roller which presses the recording material, to thermally fix the toner image onto the recording material. FIG. 1 is a cross-sectional view of a general heat roller-type fixation device, wherein 1 is a cylindrical heat roller having inside a heating means such as heater 1a, which rotates clockwise in fixation; 2 is a cylindrical pressure roller which is arranged to press on the heat roller and rotates counterclockwise in fixation; a copying paper sheet P bearing a toner image consisting of unfixed toner T is transported from the right by a transport belt 3 to the heat roller 1 and pressure roller 2 to be pressed and heated to fix the toner and then ejected leftward; 4a and 4b are separation claws for separating copying paper P, to prevent the paper from wrapping around either heat roller 1 or pressure roller 2; 6 is a felt-like oil pad into which is impregnated a mold lubricant such as silicone oil having an appropriate viscosity; and 5 is a cylindrical cleaning roller made of a brush-like fiber. The cleaning roller 5 revolves to remove the toner residue remaining on the circumferential surface of the heat roller and also serves to supply the mold lubricant properly to the heat roller.
In the prior art, the heat roller 1 has been made of a 2-5 mm-thick Al pipe as a core and has had a 200-500.mu.-thick silicone rubber or Teflon coating. The pressure roller 2 has been made of a 30 mm diameter cylindrical stainless steel as a core covered with about 5 mm-thick silicone rubber.
Requirements for such roller fixation devices include insuring fixation without offset phenomenon and providing good mold releasability without rumpling the recording paper and, when silicone oil or the like is used as a mold lubricant, providing high wear resistance of the surface of the rollers against the silicone oil, and also against the strike by separation claws and the like.
Fixation devices using the above-described conventional heat-rollers were unsatisfactory for long-term use. For example, a silicone rubber-coated heat roller, although advantageous in that it provides better fixability and fewer rumples on recording paper and a less blurred image than does a Teflon-coated heat roller, has the disadvantage that it is poor in mold releasability. An attempt has been made to coat silicone oil on the silicone rubber-coated surface to improve the mold releasability, but this tends to cause the silicone rubber to swell and to produce a permanent compression set, being accompanied by the disadvantage of significant roller-surface wear due to contact with the separation claws or the like.
For this reason, using fluororesin as the roller surface material has been proposed since it is excellent in mold releasability and does not swell in silicone oil. There have been many proposals for the application of fluororesin to the surface layer of the roller. Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 89785/1982 and 74578/1984 relate to roller fixation devices comprising a roller having a core metal provided with an elastic layer, on which is formed a fluororesin layer. Fluororesin, however, has the disadvantage that it has a high hardness and is quite different from the elastic layer thereunderneath in physical properties such as elastic modulus and when a cardboard-type thick recording paper is fixed by a fixing device using the above roller, cracks appear on the fluororesin layer's portion corresponding to a side edge of the recording paper, and thus the resin lacks durability, because of the different level between the roller surface and the thick paper surface.
Japanese Patent O.P.I. Publication Nos. 135871/1982, 5770/1983, 27175/1983 and 90673/1983 proposes roller fixation devices having a metal core covered with an elastic layer, on which is placed a fluororesin-containing fluororubber layer. These proposals utilize the phenomenon that, when a fluororesin-containing fluororubber is coated and heated to form the roller surface, the fluororesin comes out on the surface, and a fluororesin layer is formed. In this instance, the adherence of the surface fluororesin layer with the base material is excellent, and the thickness of the surface-deposited fuororesin layer is equal to or less than about 1 .mu.m. A fixation device which uses such a roller, although its mold releasability is good, is poor in durability because the surface fluororesin layer is extremely thin.